1. Field of the Invention (Technical Field)
The present invention relates generally to the field of alternating current (AC) line filters and surge suppression circuitry, and more particularly to the application of line filtering based upon a predetermined sensed inductive current at the load.
2. Background Art
The basic alternating current (AC) line filter is an inductive-capacitive (LC) filter circuit that is inserted into a power supply to filter out unwanted high-frequency interference present in the input line supply. Line filters also aid in the reduction of voltage spikes, or “surges”, as well as aid in the elimination of radio frequency (RF) interference by the power supply. Line filters are typically placed before the transformer of a power supply to prevent these unwanted signals and spikes from reaching the load.
The difficulties encountered when AC voltage surges occur in power supplies are well known. Sudden spikes or dips in the input voltage are particularly problematic to sensitive circuitry, such as digital integrated circuits, but are also problematic to larger scale circuitry. A change in the power supply output voltage, particularly sudden increases in output voltage, may damage or destroy circuit components. Most power supplies are regulated to prevent or minimize the effect of power surges as well as to maintain a constant output voltage regardless of load.
Metal oxide varistors (MOVs) and other transient voltage surge suppression (TVSS) technology, such as those manufactured by Redivolt® and Innovative Technologies, are commonly used for surge suppression. Examples of surge protection circuitry can be found in U.S. Pat. No. 6,229,682 to Mechanic, entitled, “Transient Voltage Surge Suppressor;” U.S. Pat. No. 6,055,147 to Jeffries, et al., entitled, “Apparatus for Providing Independent Over-Current Protection to a Plurality of Electrical Devices and Transient-Voltage Suppression System Employing the Apparatus;” U.S. Pat. No. 5,761,021 to Yu, entitled, “Voltage Surge Suppression Device;” U.S. Pat. No. 5,617,284 to Paradise, entitled, “Power Surge Protection Apparatus and Method;” U.S. Pat. No. 5,388,021 to Stahl, entitled, “Voltage Surge Suppression Power Circuits;” U.S. Pat. No. 4,023,071 to Fussell, entitled, “Transient and Surge Protection Apparatus;” U.S. Pat. No. 5,153,806 to Corey, entitled, “Transient Surge Suppressor and Alarm Signal Circuit;” U.S. Pat. No. 4,587,588 to Goldstein, entitled, “Power Line Transient Surge Suppressor;” U.S. patent application Pub. No. 0055186 to Wodrich, et al., entitled, “Zero Threshold Surge Suppressor;” U.S. Pat. No. 6,118,639 to Goldstein, entitled, “Fast Acting Disconnect System For Protecting Surge Suppressors and Connected Utilization Equipment From Power Line Overvoltages;” U.S. Pat. No. 5,786,974 to Zaretsky, entitled, “Apparatus For and Method of Suppressing Power Surges Utilizing Electrical Striplines;” U.S. Pat. No. 4,587,588 to Goldstein, entitled, “Power Line Transient Surge Suppressor;” U.S. Pat. No. 4,630,163 to Cooper et al., entitled, “Method and Apparatus for a Transient-Suppression Network;” U.S. Pat. No. 4,870,534 to Harford, entitled, “Power Line Surge Suppressor;” U.S. Pat. No. 5,398,150 to Standler, entitled, “Coordinated Electric Surge Suppressor with Means for Suppressing Oscillatory Transient Overvoltages;” U.S. Pat. No. 4,587,588 to Goldstein, entitled, “Power Line Transient Surge Suppressor;” U.S. Pat. No. 6,188,557 to Chaudhry, entitled, “Surge Suppressor;” and U.S. Pat. No. 4,870,528 to Harford, entitled, “Power Line Surge Suppressor.”
Additional AC filtering is achieved by including capacitive filtering in the line. Capacitive filters further aid in the electrical performance of various inductive loads such as pumps, fans, air conditioning units, refrigeration units, etc., because the capacitive reactance of the filter offsets the inductive load reactance to balance the phase variation caused by the inductive load. Examples of combination surge protection and line filtering circuitry can be found in U.S. Pat. No. 5,392,188 to Epstein, entitled, “Power Surge Transient Voltage Protection and Filtering Circuit Having Current Controlling Characteristics;” and U.S. Pat. No. 4,675,772 to Epstein, entitled, “Protector Network for A-C Equipment.”
Surge protection is useful on a continuous basis for equipment connected to a power line. However, during load non-operation periods, capacitive line filtering may not be necessary and in some cases may add interference back into the electrical supply. The capacitive reactance of the filter can cause harmonic distortion on the line because the normal inductive load is not present to absorb, or cancel, the capacitive reactance. Prior art surge suppression and filtering circuits do not address inductive load disparities that occur due to varying operating parameters of the load. It would be useful if such circuitry were able to sense an inductive current at the load and provide line filtering only when inductive current was present.
Attempts have been made to sense current within surge protection circuitry devices, but not for the purpose of controlling capacitive filtering on the line. Examples include U.S. Pat. No. 4,870,528 to Harford, which discloses a surge suppression circuit that includes a sensing circuit for sensing the charging current to a capacitor used for storing relatively high-energy surges. However, this patent addresses the need for additional energy storage capacity in a particularly high energy surge condition and allows current flow to an additional capacitor upon sensing current through a simple diode and rectifier configuration. Harford does not address the need for control of line filtering. U.S. Pat. No. 5,818,672 to Hilbe, entitled, “Ground Loop Current Suppressor” discloses a sensing transformer used for sensing ground loops in electrical equipment interconnections. The sensed ground loop current is used to induce a second current for counteracting the ground loop current. U.S. Pat. No. 6,388,852 to Bobash, entitled, “Flicker Protection Circuit” discloses a circuit for removing AC power to a load upon sensing an interruption in the power supply by an auto-reset watchdog circuit.
Problems of varying load-operating parameters have also been addressed with motor control circuitry that is used to change the running characteristics of the inductive load. Examples include variable frequency drives (VFDs), and soft starts which do not allow full transfer of power upon start-up but instead slowly ramp up power to the load. Although such control circuitry may provide significant energy savings, it often requires changing the frequency of and/or the voltage supply to the load and does not address the need for control of the application of line filtering.
The present invention overcomes the limitations of the prior art by applying AC line filtering only when the load is operational. The inventive method and apparatus monitors the inductive load via an inductive current sense loop (ICSL) circuit to control the application of line filtering according to the presence or absence of inductive current at the load. The ICSL circuit monitors current flux from an inductive load to determine if capacitive AC line filtering should be added to the circuit. If the inductive current benefits by the addition of filtering elements, then the ICSL circuit remains energized to provide such filtering elements. When the inductive load is not operational, the ICSL circuit automatically switches the line filtering elements to an “off” position, thereby removing line filtering from the system. The present invention also combines automated line filtering with surge suppression circuitry. The surge suppression circuitry is in continuous electrical communication with the power line and load for continuous provision of surge protection.